Device for the injection of fluid foaming agents into plasticized polymeric material

ABSTRACT

A device for introducing a fluid foaming agent into a plasticized polymeric material comprising, a conduit and a fluid permeable labyrinthian element interposed in the conduit for the passage of the plasticized polymeric material therethrough under pressure. Means for supplying fluid foaming agent to the fluid permeable element at a pressure greater than the pressure of the plasticized polymeric material so that the fluid foaming agent will flow through the element and into the plasticized polymeric material in a finely dispersed state.

ilnited States Patent [1 1 11] 3,792,839 Gidge Feb. 19, 1974 [54] DEVICEFOR THE INJECTION OF FLUID 3,316,335 4/1967 Snella 264/50 FOAMING AGENTSINTO PLASTICIZED POLYMERIC MATERIAL Lester Gidge, Nashua, NH.

Polysar Limited, Sarnia, Ontario, Canada Filed: Aug. 23, 1971 Appl. No.:174,117

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 4/1952 Clinefelter 259/19312/1957 Braunlich ..259/4 Primary Examiner-Robert W. Jenkins Attorney,Agent, or Firm-Cullen, Settle, Sloman & Cantor 57 ABSTRACT theplasticized polymeric material so that the fluid foaming agent will flowthrough the element and into the plasticized polymeric material in afinely dispersed state.

3 Claims, 4 Drawing Figures PATENTED I 9i974 3, 7 92'. 8 39 SHEET 1 OF 3PATENTED FEB I 9 I974 SHEET 2 BF 3 DEVICE FOR THE INJECTION OF FLUIDFOAMING AGENTS INTO PLASTICIZED POLYMERIC MATERIAL BACKGROUND OF THEINVENTION Processes for the production of foamed plastics and the likemay be divided into three main classes: mechanical, physical andchemical. Mechanical processes involve the whipping of plastic compoundsand are of limited usefulness. By far the most important and widely usedare the physical and chemical processes, both of which form thecharacteristic cellular structure by the action of foaming agents. Thesefoaming agents are materials which, when mixed with a plastic compound,react under given physical conditions to produce a foamed structure inthe material.

Chemical foaming agents decompose under the influence of heat to yieldat least one gaseous product usually nitrogen which producesthe foamingaction. There are many organic and inorganic substances which may beused, but only a few which are commercially satisfactory. Oneof the mostcommonly used in plastic forming processes is ABFA (azobisformamide).Satisfying the most important requirements of a foaming agentzdecompositiontemperature range, non toxicity, freedom from odour,staining, discolouration of the plastic, stability in storage etc., ABFAyields a good quality product. However, chemical foaming agentsgenerally are expensive; They require the separate step of mixing withthe plastic compound before plasticization, and close heat controlthroughout the process must be maintained to prevent premature foamingand also to obtain an even rate of decomposition throughout thematerial. The use of chemical foaming agents therefore involves highproduction costs and, in consequence, a more expensive product.

Physical foaming agents are materials, usually liquid or gaseous, whichchange their physical state during the foaming operation. For example:compressed gases expand when the pressure is released, and volatileliquids develop cells in the plastic material when they change fromliquid to gas. The volatile liquid agents are presently more widely usedby industry than gaseous agents. Liquid at or near ambient temperatureand pressure, their boiling points are usually below 1 C at atmosphericpressure. The group includes aliphatic hydrocarbons such as pentane.Inexpensive and of low toxicity they are, nevertheless, highlyflammable. Chlorinated hydrocarbons such as methyl chloride, widely usedin the manufacture of foamed polystyrene, present some toxicityproblems.

The use of gaseous foaming agents in direct gassing processes wouldappear to offer many advantages. Inert gases such as nitrogen will notdegrade thermoplastic compounds in any way, they can be introduceddirectly into plasticized material under pressure, and require nofurther decomposition, they are safe to use, and are relatively cheap.Their prime disadvantage lies in the difficulty of obtaining an evendistribution of the gas in minute bubbles throughout the plasticizedmaterial to produce a uniform foam structure.

- Gaseous foaming agents are generally introduced into a decompressionzone or zone of reduced pressure in a screw-type extruder. This zone,usually formed by reducing the root or stem diameter of the screw, maybe at any point along the barrel downstream from a point where thegranular feed material has become plasticized. Since the'gas is usuallyintroduced through an open port or ports in the extruder barrel, thereis always the possibility of plasticized material entering and blockingthese ports if the pressure differential between gas and plasticundergoes a reversal for any reason. This crude way of introducing gasinto plasticized material, coupled with the fact that the screw-typeextruder is not a good mixing device, results in uneven distribution ofthe gas in random size bubbles throughout the melt. The resulting foamstructure is, therefore, not uniform in density and may contain quitelarge voids. ln injection moulded structural foam parts, this results inweakened areas and, to some extent, a poor surface finish.

SUMMARY OF THE INVENTION In order to overcome these primarydisadvantages associated with the direct gassing process, the presentinvention contemplates the injection of gaseous foaming agents intoplasticized material through a fluid permeable element which presents aslarge a diffusion surface as possible to the material stream. The fluidpermeable element must, at the same time, be permeable to the gaseous(or liquid) foaming agents contemplated but impermeable to hotthermoplastic under very high pressure. It has been discovered thatcertain sintered materials, particularly sintered iron materials arewell suited to this purpose. A particular grade of material commerciallyavailable and used in the present invention is an Iron/Nickel materialcontaining 3.5 percent Ni and 0.75 percent C, density 7.18 grams per ccand a theoretical percentage density of 92.8 percent; Other sinteredmaterials of densities in close proximity to this would obviously besuitable and, in possessing other physical properties, might be moresuitable. Being machinable after sintering as well as mouldable beforesintering, structures in many shapes and forms can be produced.

It will be readily seen that the use of such fluid permeable elements inthe form of annular inserts in the walls of extruder barrels, conduits,extruder stems or heads, would be a beneficial improvement over openoutlets in gas supply systems in preventing ingress of plastic materialinto the system under all operating and non-operating conditions.However, the present invention further contemplates the formation of alabyrinthian structure interposed in the melt stream wherein the wallsof the passages therethrough contain or are substantially all diffusionsurfaces. The advantages stemming from such structure are several: In anextruder, apart from evenly dispersing the gas in tiny bubblesthroughout the melt, the structure also acts as a breaker platesupporting a screen pack if desired and,

as such, provides a back pressure inthe extruder. The

bifurcated passages of the preferred embodiment serve further to mix andhomogenize-the melt.

A preferred embodiment of the invention to be described includes aporous element permeable to gas and non-viscous fluids but impermeableto plasticized thermoplastic material and the like, and comprising aplurality of stacked,.individually perforated, sintered metal plates.The perforations of adjacent plates are offset with respect to eachother to define a labyrinthian path and prevent a straight-through flowof material through the stacked plates. The stack is housed .within acylindrical housing (such as the barrel of a screw-type extruder) sothatit is interposed in the melt stream and is supplied with a gaseous fluidsuch as nitrogen through suitable ducting. The pack is further supportedaxially against the compression forces of the melt stream by astructural member perforated to cooperate with an adjacent sinteredmetal plate. A finemesh screen of the type associated with aconventional breaker plate may also be used with the invention ifrequired.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of theinvention is shown in the following drawings in which like referencecharacters are used to denote like parts throughout the several viewsand in which:

FIG. 1, is a diagrammatic illustration of an extrusion machine showingthe location of the invention;

FIG. 2, is a detailed side elevation, in section, of the portion of theextruder including the invention shown in FIG. I;

FIG. 3, is a cross-sectional view of the invention taken along the line3-3 in FIG. 2, and with part of one of the perforated plates cut away toshow the relationship of hole patterns in adjacent plates; and

FIG. 4, is an exploded view of the stack of perforate platesconstituting the porous element.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly toFIG. 1, a single screw, extrusion machine is generally indicated athaving a barrel 11, a bore 1 la, and'a flighted screw 12 for rotationwithin the barrel by a prime mover (not shown). A hopper 13 at one endof the barrel feeds granular plastic material to the screw 12. At thedelivery end of the barrel 11 a porous element of the inventiongenerally indicated at 14 is housed in a short cylindrical housing 15attached to the barrel by any well-known means such as split C-ringclamps 16 (see FIG. 2) so as to be substantially an extension of thebarrel. A nozzle 17 is similarly attached to the delivery end of housing15. A screen pack (not shown) such as is often associated with aconventional breaker plate in extruders may be placed ahead or upstreamof the porous element if desired. Whether a screen pack is provided ornot, the porous element with its labyrinthian paths for the passage ofplasticized material therethrough creates sufficient resistance toprovide a back-pressure in the melt against which the screw can work,plasticize and homogenize the material.

Now referring more particularly to the remaining Figures, it will beseen that housing 15 is spigotted at 18 to the delivery end of extruderbarrel 11 which is shown in part. The housing is provided with a bore 19equal to or slightly larger than bore 1 1a of the extruder. Acounterbore 20 in the main portion of the housing 15 forms a narrowshoulder 21 with bore 19 and a screwthreaded counterbore 22 is adaptedto receive a ringnut 23 to be described later.

Supported within the counterbore 20 and bearing against shoulder 21 is aperforated, disc-shaped support plate 24 which is a structurally strongmember made from a material such as high tensile steel. The primefunction of plate 24 is to support the porous element 14 against thecompressive force of the plastic material in the extruder. Plate 24 isperforated in a regular grid pattern by perforations 25 on pitchcentresZD apart,

' face 26 of the plate is plain; the perforations are countersunk as at27 in a downstream face 28 of the plate. Countersinks 27 are of such asize that there is a point of contiguitybetween adjacent countersinks.This promotes good outflow characteristics from the perforations andreduces the size of dead spots which would otherwise cause a hang up ofplasticized material which would ultimately degrade then break away andpollut fresh flowing material.

Abutting plate 24 in counterbore 20 is a stack of four porous diffusionplates designated 30a, 30b, 30c and 30d of sintered ironinickelmaterial, so designated according to their order of assembly in housing15. Diffusion plate 30a is provided with perforations 31a co-extensivewith perforations 25 in support plate 24. A plain downstream face 32(FIG. 2) of the plate abuts face 26 of plate 24. The perforations 31ahave countersinks 33 in an upstream face 34 of the plate to provide agood inflow for the plastic, reduce the size of dead spots between theperforations and, more importantly, to provide the labyrinthian pathsthrough the porous element as will be described later herein. Acontinuous U shaped channel 35a is formed in the peripheral edge ofplate 30a and a series of ducts 36a (see FIGS. 2 and 3) extendstransversely through the disc between consecutive rows of perforations31a with each duct beginning and terminating in channel 35.. The purposeof ducts is to ensure a supply of gas or fluid foaming agent to thecentral area of the diffusion plate. Al though shown running parallel inone direction only, it may be desirable to have a second series runningnormal to and interlinking the first series for even better distributionof fluid within the plate.

Diffusion plate 30b is placed ahead or upstream of plate 30a. Preciselysimilar in size to plate 300, plate 30b is also provided with a similarperipheral channel 35b and transverse ducts 36b. Perforations 31b arecountersunk on both faces of the plate, again to the point ofcontiguity. However the prime difference between plates 30a and 30b liesin the positional relationship between their respective grid patterns.This relationship will be quite clearly understood by reference to FIG.3, where it will be seen that the centres of perforations 31b are offsetone half of the pitch in both horizontal and vertical directions withrespect to perforations 31a. 1

Diffusion plates 30a and 30b together form a sub-unit of porous element14. That is to say, they satisfy the secondary function of the inventionwhich is to divide and re-combine discrete streams of material forgreater homogenization of the plastics material and breakdown of theentrained gas bubbles as will be described under the section inOperation. Porous element 14 in the preferred embodiment utilizes twosub-units wherein.

diffusion plate 300 has a similar perforation pattern to plate 300 andplate 30d has a similar pattern of perforations to 30b. It will berealized that since there are but two perforation patterns, any numberof individual diffusion plates used alternately may comprise a completeelement according to theinvention. To prevent leakage, diffusion platesurfaces are ground and lapped flat. In addition, it may be desirable tosilversolder or braze the plates together to further prevent leakagebetween them and into the gas system, since the pressures in the plasticmaterial may be in the region of 5000 p.s.i. The

porous element 14 is retained in the counterbore by ring nut 23 in awell known manner.

A series of four annular channels 37 is machined in counterbore 20, eachchannel 37 cooperating with a peripheral channel such as 35a indiffusion plate 30a of each of the diffusion plates of the porouselement. Channels 37 are interconnected by three axially directedchannels 38a, 38b and 38c which are angularly disposed around thecounterbore 20. A threaded pipe union 39 set in housing 15 communicatesthrough duct 40 with axial channel 38a. Exteriorly of the housing 15,pipe union 39 is connected to a pressurized source of fluid foamingagent (not shown) but which in the preferred case is gaseous nitrogen.

In its function as a means for injecting gaseous foaming agent intoplasticized material, gas (in the present case nitrogen) at a pressurewhich is greater than the pressure of the material itself is received atunion 39 from an outside source. The gas passes into housing 15 alongduct 40 to 38a and from thence to channels 37 with channels 38b and 38bensuring equal distribution thereto. Each annular channel 37 registersindividually with a peripheral edge channel, such as 35a, in eachdiffusion plate 30a, 30b, 30c and 30d. Gas is therefore delivered toeach series of transverse ducts, such as 36a, in each diffusion plateand permeates the sintered material from which they are made. Thelabyrinthian passages formed by the perforations and their countersinksthrough which the plastic material is caused to pass are definedentirely by diffusion surfaces from which the gas continuously exudesunder pressure.

A full understanding of the paths followed by the plastic materialthrough the porous element will be gained by first referring to FIG. 2.Plastic material passes from the extruder barrel 11 through the ring nut23 and enters the perforations of plate 30d. By virtue of the offsettingof the perforations and countersinks in plate 300, the material streamsdivide as they pass through the perforations in plate 300.Re-combining'to pass through plate 30b they divide again through plate30a. However, it must be remembered that the perforations in successiveplates areoffset in two mutually perpendicular directions. In followingthe paths of material through given perforations as indicated by thearrows in FIG. 4, it will be realized that except at peripherallydisposed perforations the division at intermediate stages is fourfold.

Each division and re-combination of discrete streams of plasticeffectively mixes and homogenizes the material a little further at eachstage. Since all the internal surfaces contacted by the plastic in itspassage through element 14 are diffusiorisurfaces exuding minute bubblesof gas, the gas is finely dispersed throughout the material whichfinally re-combines into one stream on passing through the perforationsof support plate 24 into bore 19.

Although the foregoing invention has been described in some detail byway of illustration and example for the purposes of clarity andunderstanding, it is understood that certain changes-and modificationsmay be made within the spirit of the invention and scope of the appendedclaims.

What is claimed is:

1. A device for the injection of a fluid foaming agent into aplasticized polymeric material comprising a housing defining an axiallyelongated conduit for the passage of said polymeric materialtherethrough, at least two diffusion plates fixed in the conduit toextend substantially normal to the axis of the conduit, said two platesincluding a first diffusion plate of sintered material having axialperforations therethrough arranged in a grid pattern, and internal fluiddistribution ducts within the plate and communicating with the peripheryof the plate,

and a second diffusion plate of sintered material having axialperforations therethrough arranged in a grid pattern which is offsetwith respect to the grid pattern of said first diffusion plate, saidsecond diffusion plate also having internal fluid distribution ductscommunicating with the periphery of said second diffusion plate,

and supply means for conducting said fluid through said housing intocommunication with the peripheries of the diffusion plates.

2. A device for the injection of a fluid foaming agent into aplasticized polymeric material comprising a housing defining an axiallyelongated conduit for the passage of said plasticized polymeric materialtherethrough, I

a plurality of axially abutting diffusion plates extending across theconduit to lie substantially normal to the axis of the conduit, each ofsaid diffusion plates having axial perforations therethrough arranged ingrid patterns, the grid patterns of axially adjacent plates being offsetrelative to one another to define an overall labyrinth flow path throughthe plates, said plates each having a peripheral flow passage andtransverse fluid distribution ducts communicating with the peripheralflow passage and extending across the transverse extent of the plateintermediate the perforations therein, and

means for supplying said fluid foaming agent to the peripheral flowpassages of said plates.

3. A device for injection of a fluid foaming agent into a plasticizedpolymeric material comprising a housing defining a conduit for thepassage of said polymeric material therethrough,

a structurally strong perforate support plate traversing the conduit infixed-relation therewith wherein the perforations are in a grid patternof regular pitch;

a porous element in the conduit in series with the said support platecomprising,

a first diffusion plate of sintered material having perforationscoextensive with the perforations in said support plate wherein saidperforations are countersunk to a point of contiguity in a surfaceremote from the wall support plate and having fluid distribution ductswithin the plate, and

a second diffusion plate of-sintered material having perforations on agrid pattern which is offset onehalf pitch with respect to the gridpattern of said first plate wherein said perforations are countersunk toa point of contiguity in both surfaces of the plate and having fluiddistribution ducts within the plate;

means for retaining the said porous element within the conduit, and

supply means for the distribution of said fluid peripherally within thehousing to the porous element.

1. A device for the injection of a fluid foaming agent into aplasticized polymeric material comprising a housing defining an axiallyelongated conduit for the passage of said polymeric materialtherethrough, at least two diffusion plates fixed in the conduit toextend substantially normal to the axis of the conduit, said two platesincluding a first diffusion plate of sintered material having axialperforations therethrough arranged in a grid pattern, and internal fluiddistribution ducts within the plate and communicating with the peripheryof the plate, and a second diffusion plate of sintered material havingaxial perforations therethrough arranged in a grid pattern which isoffset with respect to the grid pattern of said first diffusion plate,said second diffusion plate also having internal fluid distributionducts communicating with the periphery of said second diffusion plate,and supply means for conducting said fluid through said housing intocommunication with the peripheries of the diffusion plates.
 2. A devicefor the injection of a fluid foaming agent into a plasticized polymericmaterial comprising a housing defining an axially elongated conduit forthe passage of said plasticized polymeric material therethrough, aplurality of axially abutting diffusion plates extending across theconduit to lie substantially normal to the axis of the conduit, each ofsaid diffusion plates having axial perforations therethrough arranged ingrid patterns, the grid patterns of axially adjacent plates being offsetrelative to one another to define an overall labyrinth flow path throughthe plates, said plates each having a peripheral flow passage andtransverse fluid distribution ducts communicating with the peripheralflow passage and extending across the transverse extent of the plaTeintermediate the perforations therein, and means for supplying saidfluid foaming agent to the peripheral flow passages of said plates.
 3. Adevice for injection of a fluid foaming agent into a plasticizedpolymeric material comprising a housing defining a conduit for thepassage of said polymeric material therethrough, a structurally strongperforate support plate traversing the conduit in fixed relationtherewith wherein the perforations are in a grid pattern of regularpitch; a porous element in the conduit in series with the said supportplate comprising, a first diffusion plate of sintered material havingperforations coextensive with the perforations in said support platewherein said perforations are countersunk to a point of contiguity in asurface remote from the support plate and having fluid distributionducts within the plate, and a second diffusion plate of sinteredmaterial having perforations on a grid pattern which is offset one-halfpitch with respect to the grid pattern of said first plate wherein saidperforations are countersunk to a point of contiguity in both surfacesof the plate and having fluid distribution ducts within the plate; meansfor retaining the said porous element within the conduit, and supplymeans for the distribution of said fluid peripherally within the housingto the porous element.